I think for a given loop size, .325 will have more teeth to sharpen than 3/8. The .325 supposedly runs a little smoother since there are more teeth and they take a smaller bite, but I haven't noticed a big difference. Some guys do timed cuts comparing the two on small saws and the .325 comes out a little faster, but to me, the 3/8 feels faster (on the 261 and the 346 anyway). Less teeth to sharpen with 3/8 and the chain should last a little longer since the teeth are larger and there is more material there to get more sharpenings. If you stay with 3/8, bars can be easily swapped between saws w/o having to change rim sprockets, and you only need 1 file size - overall less inventory in parts. The 026 is a 50cc saw, but it's a fair amount weaker than the 261 and 346 in my experience - it may benefit more by running the smaller chain, but I only run 3/8 on my 346 and 261 and I much prefer it that way.
Remember that whatever you get (if it's new), you may be initially less impressed with it as many of the newer saws seem to take more time to break in - maybe 10-15 tanks - if this is the case, just give it that time to break in and the power will come on. If I were in your shoes, I'd opt for the 261c with 3/8" chain, 16" or 18" bar. Do a muffler mod as soon as you can or have someone else do one for you. The Mtronic will automatically compensate for that and even for porting. If you are interested in getting it ported, it's roughly $250 plus shipping both ways (figure $25 each way), so about $300 total. Mastermind (Randy Evans), Bsnelling (Brad Snelling), Tlandrum (Terry Landrum), Moody are all sponsors that port and would do a good job IMO. For most saws, I believe they usually see 30%-40% gains from porting. Look up some of the threads with videos and you can see comparisons of power/speed from stock until after ported - quite the dramatic difference.
Waylan